Composite constraints are composed of multiple columns. Simple constraints are composed of a single column. Unique constraints guarantee that no rows with duplicate column values exist in a table.Ī unique constraint can be simple or composite. A candidate key is an attribute or a set of attributes such as columns that uniquely identify each row in the relation or table data. Unique constraints should be used for all candidate keys. In SQL Server, you can use user-defined functions in constraints to access other rows, tables, or even databases. In ANSI SQL, check constraints can’t access other rows as part of the expression. In this case, it is known as a table-level check constraint. A single check constraint may apply to multiple columns. You can assign multiple check constraints to a single column. For check constraints, an expression that evaluates to UNKNOWN is functionally equivalent to TRUE because the value is permitted by the constraint. For example, in a WHERE clause, a logical expression that evaluates to UNKNOWN is functionally equivalent to FALSE and the row is filtered out. Let’s look at the example below.Check constraint expressions behave differently than predicates in other query clauses. That is, the unique key is invalid for the NULL value. However, the NULL value breaks the unique key constraint. ![]() Unlike primary keys, unique keys allow columns within them to accept NULL values. Note that the value in the key_name column is the constraint name. | user_hobby | 0 | unique_user_hobby | 2 | hobby | A | 0 | NULL | NULL | | BTREE | | | YES | NULL | | user_hobby | 0 | unique_user_hobby | 1 | user_id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL | | user_hobby | 0 | PRIMARY | 1 | hobby_id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL | | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | Here is the syntax to define a column as a unique column when creating a table: You can define a unique key when creating a table or add a unique key by modifying the table after creating the table. To define a unique key, please use the UNIQUE keyword. A unique key corresponds to a unique index. ![]() Primary key columns cannot contain NULL values, while unique key columns can contain NULL values. The unique key is generally used to constrain the uniqueness of business-related data. The primary key generally adopts a value that has nothing to do with the business, such as auto-increment, UUID, etc. In the daily statistical report, the three columns of year, month and day are used as the combined unique key.Ĭompared with the primary key, the primary key is used to represent the identification of a row.The order number column in the order table is unique.The Product ID column in the Product table is unique.There are login or email columns in the user table that are unique.We use unique key constraints in many systems, such as: A unique key is also a commonly used constraint to ensure that the values in one or more columns in a table are unique. In MySQL, we can define many constraints on a table, such as primary key constraints, foreign key constraints.
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